1. Field of the Invention
The present invention relates to the field of flat display screens, and more specifically to so-called cathodoluminescent screens, an anode of which supports phosphor elements likely to be excited by electron bombarding. The present invention more specifically applies to screens of field-effect type, in which the electron bombarding canes from microtips supported by a screen cathode.
2. Discussion of the Related Art
FIG. 1 shows an example of a conventional structure of a flat color microtip screen of the type to which the present invention relates. Such a screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of the microtips. Cathode 1 is placed opposite to a cathodoluminescent anode 5, a substrate 6 of which, for example, made of glass, generally forms the screen surface.
The operating principle and a specific embodiment of a microtip screen are described, for example, in U.S. Pat. No. 4,940,916 of the Commissariat à 1 xe2x80x2 Energie Atomique.
Cathode 1 is generally arranged in columns and is formed, on a substrate 10, for example, made of glass, of cathode conductors arranged in meshes from a conductive layer. Microtips 2 are generally made on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh, without showing the cathode conductors. Cathode 1 is associated with grid 3, comprising row conductors. Gate 3 is deposited on the cathode plate with an interposed insulating layer 12. The intersection of a grid row and of a cathode column generally defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2. The electrons are then attracted by phosphor elements 7 of anode 5, if said elements are properly biased. In the case of a color screen such as illustrated in FIG. 1, anode 5 is, for example, provided with alternate strips of phosphor elements 7r, 7g, 7b, corresponding to each of the colors (Red, Green, Blue). The strips may be separated from one another by an insulator 8. The phosphor elements are deposited on electrodes 9, for example, formed of corresponding strips of a conductive layer (transparent if the anode forms the screen surface), for example, indium and tin oxide (ITO). The sets of red, green, blue strips are for example alternately biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed to the phosphor elements 7 facing each of the colors.
In the case, not shown, of a monochrome screen, the anode supports phosphor elements of same color arranged in a single plane or in two sets of separately-biased alternate strips.
Other cathode-grid and anode structures than those described hereabove may be encountered. For example, the phosphor elements of the anode may be distributed in elementary patterns corresponding to the sizes of the screen pixels. The anode may further, while being formed of several sets of strips or of elementary patterns of phosphor elements, not be switched by sets of strips or patterns. All the strips or patterns then are at a same voltage, for example, by being supported by a conductive plane. The anode is then said to be xe2x80x9cunswitchedxe2x80x9d, as opposed to switched anodes where the colors are alternately biased.
The anode strips or patterns supporting phosphor elements to be excited are biased under a voltage of several hundreds, or even a few thousands, of volts with respect to the cathode. In the case of a switched anode screen having several sets of strips, the other strips are at a zero voltage. The choice of the values of the biasing voltages is linked to the characteristics of the phosphor elements and of the emissive means on the cathode side.
For an electron emission by the cathode microtips, said cathode must be submitted, with respect to grid 3, to a sufficient potential difference. Conventionally, under a potential difference on the order of 50 V between the cathode and the grid, there is no electron emission, and the maximum emission used corresponds to a potential difference on the order of 80 V. For example, the rows of grid 3 are sequentially biased to a voltage on the order of 80 V while the columns of cathode 1 are brought to respective voltages ranging between a maximum emission voltage and a no emission voltage (for example, respectively 0 and approximately 40 V). The brightness of all pixels in a row is thus determined (per color component if the anode includes several sets of strips selectively biased color per color).
FIG. 2 shows the equivalent electric diagram of a conventional pixel of a color microtip screen. It is arbitrarily assumed to be a pixel, but it should be noted that this same equivalent electric diagram corresponds to that of each emissive microtip. However, since the microtips are several thousands per screen pixel, the present description is simplified by referring to a pixel (or to a sub-pixel in the case where the grid rows are divided up per color).
The pixel microtips electrically form a current source 20, a first terminal 21 of which is connected, via a resistor 22 symbolizing the resistive layer (11, FIG. 1), to a terminal 23 of application of cathode voltage V1. The other terminal 24 of current source 20 corresponds to the tips of microtips 2 directed towards the anode symbolized by a plate 25 to which is applied a biasing voltage V5. The insulator (12, FIG. 1) between the grid and the cathode can be modeled by a capacitor 26 connecting terminal 21 of current source 20 to a grid row 28, and thus to a terminal 27 of application of a biasing voltage V3 of the grid row. Due to the holes (4, FIG. 1) made in the grid, grid row 28 is connected directly connected to the tip (current source 20).
FIGS. 3A and 3B schematically illustrate the meshing of the cathode conductors of a conventional microtip screen. FIG. 3A partially shows in top view a cathode plate of a flat screen, that is, a microtip cathode associated with a grid, and FIG. 3B is a cross-section view along line B-Bxe2x80x2 of FIG. 3A. For clarity, the limits between the different layers have been shown in top view, in a shifted way in FIG. 3A, to be made visible. It should however be noted that, except for the microtips, the edges of the different layers can be considered as being substantially vertical, their inclination being essentially due to the used deposition and etch techniques, the manufacturing of microtip screens using techniques currently used in integrated circuit manufacturing.
Several microtips 2, for example, sixteen, are arranged in each mesh 31 defined by cathode conductors 32. Although a reduced number of meshes has been shown for each pixel 33 defined by the intersection of a column 34 of cathode 1 with a line 35 of grid 3, it should be noted that the microtips are generally several thousands per screen pixel.
Cathode 1 is generally formed of layers successively deposited on substrate 10. A conductive layer, for example, formed of niobium, is deposited on substrate 10. This layer is etched according to the pattern of columns 34, each column comprising meshes 31 surrounded with cathode conductors 32. A resistive layer 11 is then deposited on cathode conductors 32. Resistive layer 11, formed, for example, of phosphorus-doped amorphous silicon, has the purpose of protecting each microtip 2 against an excess current upon its starting. Such a resistive layer 11 aims at homogenizing the electron emission of the microtips 2 of a pixel of cathode 1 and thus at increasing its lifetime. The resistive layer may be etched according to the column pattern and/or at least partially opened above the cathode conductors. An insulating layer 12, for example, made of silicon oxide (SiO2), is deposited on resistive layer 11 to insulate cathode conductors 32 from grid 3. A microtip cathode of this type is described, for example, in European patent application No. 0,696,045.
Cathode conductors 32 may be deposited on resistive layer 11 which may, as in the preceding case, be or not a full plate layer. A microtip cathode of this type is described, for example, in French patent application No. 2,722,913.
Grid 3 is, for example, formed from a niobium conductive layer, deposited on insulating layer 12. It is etched according to the pattern of lines 35 and is opened, like insulating layer 12, to form holes 4 above each microtip 2.
The conventional addressing of the cathode and of the grid of a flat microtip screen results in that each line of grid 3 is only addressed for a short time. For example, the grid lines are sequentially biased during a xe2x80x9cline timexe2x80x9d during which each column 34 of cathode 1 is brought to a voltage which is a function of the brightness of the pixel to be displayed along the current line. The biasing of the cathode columns changes for each new line. A xe2x80x9cline timexe2x80x9d corresponds to the duration of a frame divided by the number of lines of grid 3. The display of an image is performed during an xe2x80x9cimage timexe2x80x9d(for example, 20 milliseconds for a 50 Hz frequency). For a color screen with a switched anode, a xe2x80x9cframe timexe2x80x9d approximately corresponds to one third of the xe2x80x9cimage timexe2x80x9d decreased by the time required by possible anode switchings.
The conventional addressing of such a display screen causes brightness problems which are permanently attempted to be improved. The screen brightness depends on several factors, among which the anode bias voltage with respect to the cathode, the peak emission current of the microtips, the light efficiency of the phosphor elements, the emission surface area of a pixel, and the emission duration.
The anode-cathode voltage essentially depends on the height of the inter-electrode space. The emission current depends on the characteristics of the microtips, and the light efficiency depends on the phosphor elements and may vary under the influence of the electron bombarding. The emission surface area depends on the definition desired for the screen, that is, on the pixel surface. As for the emission duration, it depends on the line time, that is, on the duration during which each line is addressed in the scanning.
In such a screen, advantage is taken of the remanence of human eye to use only a small duty cycle (duration of a line addressing with respect to the frame duration).
It has already been suggested to lengthen the emission duration of the tips of a pixel of the cathode-grid without adversely affecting the screen control capacity in a row or column scanning system.
For example U.S. Pat. Nos. 5,313,140 and 5,537,007 provide, in the cathode-grid plate, temporary storing elements to hold the luminance control signal during a time period longer than the addressing time of a pixel.
FIG. 4 shows an equivalent electric diagram of a microtip screen pixel provided with such a storing element. FIG. 4 should be compared with previously-described FIG. 2. Such a pixel may further be modeled as a current source 20 representing the pixel microtips and having a first terminal 21 connected to a grid line 28 via a capacitor 26 representing the capacitance of the insulator (12, FIGS. 1 and 3B) between the cathode and the grid. A second terminal 24 of current source 20 represents the tip of the microtip directed towards anode 25. Terminal 21 of source 20 is connected, via a resistor 22 symbolizing the resistive layer (11, FIGS. 1 and 3B) to a first terminal of a switch 40, a second terminal of which is connected to a terminal 23 for addressing the considered column.
Without modifying the principle of addressing by scanning the rows and applying a luminance control signal on the screen columns, it can be seen that the embodiment of FIG. 4 still suffers from a pollution of the power stored in capacitor 26 by the emission current.
To avoid for the storage of the luminance control signal to be disturbed by the electron emission, the luminance information is stored from the screen grid and not from its cathode.
An object of the present invention is to provide a new solution for improving the amount of electrons emitted by the microtips of a screen pixel during each frame time.
The present invention also aims at providing a solution which does not require increase of the emission surface area and thus maintains, or even improves, the screen resolution.
The present invention further aims at providing a solution which does not require modifying the microtip emission current and thus keeps conventional microtip manufacturing methods.
More specifically, the present invention provides, for each screen pixel, a transistor for isolating an element for temporarily storing the luminance control signal of the considered pixel, outside of an addressing period of this pixel, the control gate oxide of each transistor being formed in an insulating layer separating the cathode conductors from the grid conductors of the emissive areas.
According to an embodiment of the present invention, each transistor is a depletion transistor including a first contact in a same conductive level as the microtip biasing conductors, and a second contact in a conductive level in which are formed extraction grid regions, corresponding to each pixel.
According to an embodiment of the present invention, the depletion area of each transistor is formed in a semiconductor level constitutive of a resistive layer for biasing the microtips.
According to an embodiment of the present invention, the microtips are connected to a fixed voltage, the luminance control signal being applied on the extraction grid.
The present invention also provides a flat display screen of the type including a cathode with microtips for bombarding a cathodoluminescent anode, the capacitance of the storage elements associated with each pixel being a function of the number of screen lines and of the voltage between the anode and the cathode.
The present invention also provides a method for controlling a flat display screen consisting of performing a row scanning for successively biasing the row conductors and, for each row, applying a luminance control signal on each column conductor.
According to an embodiment of the present invention, the method consists of discharging all storage elements of the screen between two display frames.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings.